Non-magnetic nickel powders and method for preparing the same

ABSTRACT

Provided are non-magnetic nickel powders and a method for preparing the same. The nickel powders have non-magnetic property and a HCP crystal structure. The method include (a) dispersing nickel powders with a FCC crystal structure in an organic solvent to prepare a starting material dispersion, and (b) heating the starting material dispersion to transform the nickel powders with the FCC crystal structure to the nickel powders with the HCP crystal structure. The nickel powders do not exhibit magnetic agglomeration phenomenon. Therefore, the pastes for inner electrode formation in various electronic devices, which contain the nickel powders of the present invention, can keep the well-dispersed state. Also, inner electrodes made of the nickel powders can have a low impedance value even at high frequency band.

BACKGROUND OF THE INVENTION

This application claims priority from Korean Patent Application No.2003-22217, filed on Apr. 9, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to nickel powders and a method forpreparing the same.

2. Description of the Related Art

Nickel is a transition metal that belongs to the iron group in Period 4,Group VIII of the periodic table and is a crystalline substance withhigh melting point and excellent malleability.

Nickel powders are a particle-phase metallic nickel material. Nickelpowders can be used as, for example, a material for inner electrodes inelectronic devices such as multilayer ceramic capacitors (MLCCs), amagnetic material, an electrical contact material, a conductive adhesivematerial, or a catalyst.

Nickel is known as a representative of ferromagnetic substances.Ferromagnetic substances are those that are strongly magnetized in thedirection of a magnetic field applied, and retain magnetization evenwhen the magnetic field is removed.

When a non-magnetized ferromagnetic substance is exposed to anincreasing magnetic field, magnetization occurs slowly at an earlystage, which is called initial magnetization. Thereafter, the rate ofmagnetization increases and saturation occurs. When a magnetic field isdecreased at a saturation state, magnetization is reduced. However, thereduction course of magnetization is different from the increase courseof magnetization. Also, even when a magnetic field becomes zero,magnetization does not reach zero, which is called residualmagnetization. When the direction of a magnetic field is reversed andthe intensity of the reverse magnetic field is increased, magnetizationreaches zero and then the direction of the magnetization is reversed.Thereafter, the reverse magnetization gradually reaches a saturationstate. At this time, even when a magnetic field becomes zero,magnetization does not reach zero and reverse residual magnetizationremains, thereby creating a closed curve which does not pass through theorigin. The closed curve is called a magnetization curve. Themagnetization curve is closely related with a magnetic domain structure.

It is known that a ferromagnetic substance has an increased magneticmoment, which is a causative factor of magnetization, produced byparallel electron spins. Also, it is assumed that a ferromagneticsubstance has magnetic domains which are clusters of parallel spins.When a magnetic field is applied, magnetic domains are aligned in thedirection of the magnetic field. Even when a magnetic field is removed,the orientations of the magnetic domains are maintained for a long time,thereby generating residual magnetization. In this regard, when atemperature of a ferromagnetic substance is raised, the alignment ofelectron spins in the ferromagnetic substance is randomized by thermalmotion. As a result, the ferromagnetic substance loses ferromagnetismand is transformed into a paramagnetic substance. The temperature iscalled the Curie temperature. The magnitude of a reverse magnetic fieldnecessary to reduce the magnetization of a magnetized magnetic substanceto zero is the coercive force.

Magnetic properties of bulk nickel are as follows: about 353° C. of theCurie temperature, about 0.617 T of saturation magnetization, about0.300 T of residual magnetization, and about 239 A/m of coercive force.

Allotropes of nickel that have been known until now include metallicnickel with a face-centered cubic (FCC) crystal structure and metallicnickel with a hexagonal close packed (HCP) crystal structure.

Almost all common nickel powders are ferromagnetic substances with a FCCcrystal structure. There are very rare reports of preparation of nickelpowders with a HCP crystal structure. It has been predicted that thenickel powders with a HCP crystal structure are also ferromagneticsubstances.

Based on the Stoner theory, D. A. Papaconstantopoulos et al. predictedthat HCP nickel must be a ferromagnetic substance [D. A.Papaconstantopoulos, J. L. Fry, N. E. Brener, “Ferromagnetism inhexagonal close packed elements”, Physical Review B, Vol. 39, No.4,1989. 2. 1, pp 2526–2528].

With respect to preparation of inner electrodes for electronic devicesthat are representative application areas of nickel powders,conventional ferromagnetic nickel powders have the followingdisadvantages.

First, when nickel powders contained in pastes for nickel innerelectrode formation by a printing method exhibit magnetism, the nickelpowders are attracted to each other like magnets and agglomerated, whichrenders uniform paste formation difficult.

Second, an ultra-high frequency band is used in electronic devices withdevelopment of the mobile communication and computer technologies.However, magnetic substances have a high impedance value at such a highfrequency band.

These problems can be solved by using non-magnetic nickel powders.

SUMMARY OF THE INVENTION

The present invention provides non-magnetic nickel powders.

The present invention also provides a method for preparing non-magneticnickel powders.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is an X-ray diffraction (XRD) pattern of nickel powders accordingto an example of the present invention;

FIG. 2 is a magnetization curve of the nickel powders according to anexample of the present invention;

FIG. 3 is an XRD pattern of nickel powders according to another exampleof the present invention; and

FIG. 4 is a magnetization curve of the nickel powders according toanother example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to an aspect of the present invention, there is providednickel powders, which are particle-phase metallic nickel materials thatare non-magnetic and have a HCP crystal structure.

According to another aspect of the present invention, there is provideda method for preparing non-magnetic nickel powders with a HCP crystalstructure, which include (a) dispersing nickel powders with a FCCcrystal structure in an organic solvent to prepare a starting materialdispersion and (b) heating the starting material dispersion to transformthe nickel powders with the FCC crystal structure to the nickel powderswith the HCP crystal structure.

The present inventors found that when nickel powders of FCC phase, whichare ferromagnetic substances, are heated in an organic solvent, they aretransformed from a FCC crystal structure to a HCP crystal structure andthe nickel powders thus transformed are non-magnetic. There are nodisclosures and predictions that nickel powders in an organic solventare transformed by heating and the nickel powders thus transformed arenon-magnetic.

An X-ray diffraction (XRD) analysis result of the crystal structure ofnickel powders according to a first example of the present invention isshown in FIG. 1. FIG. 1 shows overlapping XRD peaks of nickel powdersprepared from a same starting material (i.e., FCC nickel powders with anaverage particle size of about 150 nm prepared by a liquid phasereduction method using hydrazine) with respect to a phase transitiontime. From the XRD patterns for the nickel powders at a phase transitiontime of 1 to 24 hours, it can be seen that nickel powders of the presentinvention have a HCP crystal structure. The XRD pattern represented by 0hours is for the starting material with FCC phase.

The XRD pattern represented by 0 hours shows (111), (200), and (220)peaks at two (2) theta values of 44.5, 51.8, and 76.4. The (200) and(220) peaks at two theta values of 51.8 and 76.4 indicate that thestarting material is of FCC phase.

The (200) and (220) peaks at two theta values of 51.8 and 76.4 thatappear in the XRD pattern represented by 0 hours are gradually weakenedwith a phase transition time, and completely disappear in the XRDpattern represented by 4 hours. The XRD patterns after 4 hours show(010), (002), (011), (012), and (110) peaks at two theta values of 39.1,41.5, 44.5, 58.4, and 71.2. These peaks indicate that correspondingnickel powders are of HCP phase. In the XRD patterns represented by 1,2, and 3 hours, the peaks that represent FCC phase and HCP phasecoexist. This means that corresponding nickel powders are a mixture ofFCC nickel powders and HCP nickel powders. That is, in this example, thestarting material is completely transformed after 4 hours.

FIG. 3 is an XRD analysis result of the crystal structure of nickelpowders according to a second example of the present invention. FIG. 3shows overlapping XRD peaks of nickel powders prepared from a samestarting material (i.e., FCC nickel powders with an average particlesize of about 150 nm (NF1A, Toho, Japan)) with respect to a phasetransition time. From the XRD patterns of the nickel powders at a phasetransition time of 1 to 24 hours, it can be seen that nickel powders ofthe present invention have a HCP crystal structure. The XRD patternrepresented by 0 hours is for the starting material.

Referring to FIG. 3, like in FIG. 1, the (200) and (220) peaks at twotheta values of 51.8 and 76.4 that appear in the XRD pattern representedby 0 hours are gradually weakened with a phase transition time andcompletely disappear in the XRD pattern represented by 4 hours. That is,in this embodiment, phase transition is completed after 4 hours.

The completion time of phase transition may vary according to processparameters such as the type of the organic solvent, heating temperature,and the particle size of the starting material, but is not an importantfactor in the present invention. The important matter is that nickelpowders prepared by the phase transition have a HCP crystal structure.

Nickel powders of the present invention are non-magnetic can be seenfrom magnetization curves shown in FIGS. 2 and 4.

FIG. 2 is a magnetization curve of the nickel powders according to thefirst example of the present invention. FIG. 2 shows overlappingmagnetization curves of nickel powders prepared from a same startingmaterial (i.e., FCC nickel powders with an average particle size ofabout 150 nm prepared by a liquid phase reduction method usinghydrazine) with respect to a phase transition time. From themagnetization curves of the nickel powders at a phase transition time of1 to 24 hours, it can be seen that the magnetization levels of thenickel powders decrease with increasing phase transition time. Themagnetization curve represented by 0 hours is for the starting material.

In FIG. 3, the magnetization curves for the nickel powders at a phasetransition time of 4 hours or more are unidentifiable due tosuperposition. It appears that non-magnetization capability of thenickel powders is almost completed after phase transition for 4 hours.Variations in residual magnetization and saturation magnetization of thenickel powders of the first example with respect to a phase transitiontime are summarized in Table 1 below.

TABLE 1 Phase Transition Residual Magnetization Saturation MagnetizationTime (hours) (emu/g) (emu/g) 0 (FCC phase) 5.25 24.49 1 2.42 13.13 21.29 6.165 3 0.481 2.066 4 0.194 0.784 5 0.100 0.392 6 0.0669 0.250 70.0510 0.193 8 0.0301 0.137 9 0.0255 0.103 10  0.0210 0.0857 12  0.01960.0791 18  0.00822 0.0641 24  0.00753 0.0543

FIG. 4 is a magnetization curve of the nickel powders according to thesecond example of the present invention. FIG. 4 shows overlappingmagnetization curves of nickel powders prepared from a same startingmaterial (i.e., FCC nickel powders with an average particle size ofabout 150 nm (NF1A, Toho, Japan)) with respect to a phase transitiontime. From the magnetization curves of the nickel powders at a phasetransition time of 1 to 24 hours, it can be seen that the magnetizationlevels of the nickel powders decrease with increasing phase transitiontime. The magnetization curve represented by 0 hours is for the startingmaterial.

Variations in residual magnetization and saturation magnetization of thenickel powders of the second example with respect to a phase transitiontime are summarized in Table 2 below.

TABLE 2 Phase Transition Residual Magnetization Saturation MagnetizationTime (hours) (emu/g) (emu/g) 0 (FCC phase) 3.467 40.20 1 1.742 19.92 20.879 9.91 3 0.683 3.35 4 0.241 1.02 24  0.0120 0.0721

As seen from Tables 1 and 2, the present invention can provide nickelpowders with the residual magnetization of about 2 emu/g or less,preferably about 1 emu/g or less, and more preferably about 0.2 emu/g orless. Also, the present invention can provide nickel powders with thesaturation magnetization of about 20 emu/g or less, preferably about 10emu/g or less, and more preferably about 1 emu/g or less.

There are no particular limitations on the average particle size ofnickel powders of the present invention. The average particle size ofnickel powders of the present invention may be substantially the same asthat of the FCC nickel powders that are used as a starting material.Generally, the average particle size of nickel powders of the presentinvention may be in a range of about 30 to 800 nm. In particular, it maybe preferably about 30 to 300 nm when the nickel powders are used inpastes for inner electrode formation in MLCCs. The upper limit and lowerlimit of the average particle size of nickel powders may vary accordingto application areas of the nickel powders. The limitation reasons ofthe average particle size of nickel powders are omitted herein.

Hereinafter, a method for preparing non-magnetic nickel powders with aHCP crystal structure will be described in detail.

A method for preparing non-magnetic nickel powders with a HCP crystalstructure include (a) dispersing nickel powders with a FCC crystalstructure in an organic solvent to prepare a starting materialdispersion and (b) heating the starting material dispersion to transformthe nickel powders with the FCC crystal structure to the nickel powderswith the HCP crystal structure.

The reason for the phase transformation of nickel powders by heating inan organic solvent has not been elucidated, but it seems that metallicnickel is dissolved in the organic solvent and then is recrystallized orreduced. Even though the exact mechanism of the phase transition has notbeen elucidated, the effectiveness of the present invention would not beaffected.

The organic solvent may be a glycol based organic solvent. Examples ofthe glycol based organic solvent include ethyleneglycol,propyleneglycol, diethyleneglycol, triethyleneglycol, dipropyleneglycol,hexyleneglycol, and butyleneglycol.

The nickel powders of FCC phase used as a starting material arecommercially available or can be obtained by one of known nickel powderpreparation methods. There are no particular limitations on the averageparticle size of the nickel powders of FCC phase used as a startingmaterial. FCC nickel powders with an average particle size and particlesize distribution that are generally required in the related applicationareas may be used. As the particle size of the starting materialdecreases, phase transition may be promoted, and as the particle size ofthe starting material increases, phase transition may be retarded. Thus,it is preferable to raise the heating temperature for the startingmaterial with a large particle size.

In step (a), there are no particular limitations on the content of thenickel powders of FCC phase in the dispersion provided that the nickelpowders can be well dispersed in the organic solvent. However, if thecontent of the nickel powders of FCC phase is too low, the organicsolvent may be consumed excessively. On the other hand, if it is toohigh, the nickel powders may not be well dispersed. In this regard, thecontent of the nickel powders of FCC phase may be in a range of about0.01 to about 30 wt %.

In a case where a material which is solid at room temperature such as2,3-butyleneglycol with a melting point of 34.4° C. is used as theorganic solvent, step (a) may be preformed by heating at a temperatureabove the melting point of the organic solvent.

In step (b), if the heating temperature for the dispersion is too low,the phase transition from FCC to HCP for the nickel powders may not becompleted. Even if the heating temperature is too high, phase transitioneffect may be saturated. And, the organic solvent used may be thermallydecomposed. In this regard, the heating temperature for the dispersionmay be in a range of about 150 to about 380° C.

In an embodiment of a method of the present invention that uses anairtight reaction vessel provided with a reflux cooling apparatus forthe organic solvent, it is preferable to set the heating temperature forthe dispersion to about the boiling point of the organic solvent. If theheating temperature is excessively lower than the boiling point of theorganic solvent, phase transition may not be completed. On the otherhand, if it is excessively higher than the boiling point of the organicsolvent, there arises a problem in that a reaction vessel resistant tohigh pressure must be used. In this regard, it is preferable to set theheating temperature to a range of the boiling point of the organicsolvent ±5° C. More preferably, the dispersion may be heated so that theorganic solvent of the dispersion comes to a boil.

There are no particular limitations on a phase transition time, i.e., atime for which the dispersion is heated for phase transition. The phasetransition may be continued for a sufficient time so that substantiallyall of the nickel powders of FCC phase are transformed to the nickelpowders of HCP phase. The phase transition time according to concretereaction conditions can be easily determined.

When the phase transition is completed, the nickel powders of HCP phaseare separated from the dispersion by washing and drying that aregenerally used in preparation of nickel powders.

The nickel powders of HCP phase prepared according to the method of thepresent invention have non-magnetic property.

Hereinafter, the present invention will be described more specificallyby Examples. However, the following Examples are provided only forillustrations and thus the present invention is not limited to or bythem.

EXAMPLE 1

Nickel powders of FCC phase with an average particle size of about 150nm were prepared by a liquid phase reduction method using hydrazine. TheXRD pattern and magnetization curve for the nickel powders of FCC phasethus prepared are respectively shown in FIG. 1 (represented by 0 hours)and FIG. 2 (represented by 0 hours).

100 g of the nickel powders of FCC phase were dispersed in 1 L ofdiethyleneglycol to prepare a starting material dispersion. Thedispersion was placed in a reactor provided with a reflux coolingapparatus and then heated so that diethyleneglycol came to a boil. Atthis time, the heating temperature for the dispersion was about 220° C.

The XRD patterns and magnetization curves for the nickel powders at aphase transition time of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 18hours are respectively shown in FIGS. 1 and 2.

The XRD pattern analysis for the nickel powders was performed usingX'PERT-MPD system (Philips). The magnetization curves for the nickelpowders were measured using MODEL4VSM 30 kOe (DMS).

EXAMPLE 2

NF1A (Toho, Japan) was used as a starting material. NF1A is nickelpowders of FCC phase prepared by a vapor phase method and has an averageparticle size of about 150 nm. The XRD pattern and magnetization curvefor NF1A are respectively shown in FIG. 3 (represented by 0 hours) andFIG. 4 (represented by 0 hours).

100 g of NF1A was dispersed in 1 L of diethyleneglycol to prepare astarting material dispersion. The dispersion was placed in a reactorprovided with a reflux cooling apparatus and then heated so thatdiethyleneglycol came to a boil. At this time, the heating temperaturefor the dispersion was about 220° C.

The XRD patterns and magnetization curves for NF1A at a phase transitiontime of 1, 2, 3, 4, and 24 hours are respectively shown in FIGS. 3 and4.

As apparent from the above description, the present invention providesnon-magnetic nickel powders. The nickel powders have a HCP crystalstructure.

The nickel powders of the present invention do not exhibit magneticagglomeration phenomenon. Therefore, the pastes for inner electrodeformation in various electronic devices, which contain the nickelpowders of the present invention, can keep the well-dispersed state.Also, inner electrodes made of the nickel powders can have a lowimpedance value even at high frequency band.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. Non-magnetic nickel powders having an hexagonal close packed (HCP)crystal structure wherein the average particle size of said non-magneticnickel powders is in the range of 30 to 800 nm.
 2. The non-magneticnickel powders of claim 1, wherein the residual magnetization of thenon-magnetic nickel powders is 2 emu/g or less.
 3. The non-magneticnickel powders of claim 1, wherein the residual magnetization of thenon-magnetic nickel powders is 1 emu/g or less.
 4. The non-magneticnickel powders of claim 1, wherein the residual magnetization of thenon-magnetic nickel powders is 0.2 emu/g or less.
 5. The non-magneticnickel powders of claim 1, wherein the saturation magnetization of thenon-magnetic nickel powders is 20 emu/g or less.
 6. The non-magneticnickel powders of claim 1, wherein the saturation magnetization of thenon-magnetic nickel powders is 10 emu/g or less.
 7. The non-magneticnickel powders of claim 1, wherein the saturation magnetization of thenon-magnetic nickel powders is 1 emu/g or less.
 8. The non-magneticnickel powders of claim 1, wherein the average particle size of thenon-magnetic nickel powders is in a range of 30 to 300 nm.
 9. Thenon-magnetic nickel powders of claim 1, wherein the non-magnetic nickelpowder is prepared by a method for preparing non-magnetic nickel powderswith a HOP crystal structure, comprising: (a) dispersing nickel powderswith a FCC crystal structure in an organic solvent to prepare a startingmaterial dispersion; and (b) heating the starting material dispersion totransform the nickel powders with the FCC crystal structure to thenickel powders with the HCP crystal structure.